Rotary kilns having multiple-chamber heat exchangers are well known in the art for purposes such as calcining of calcium carbonate in the production of commercial lime. Representative multiple-chamber heat exchanger rotary kilns are shown in U.S. Pat. No. 2,889,143 issued to W. A. Reaney et al. and U.S. Pat. No. 3,201,100 issued to P. Dussossoy. These patents in particular disclose three- and four-chamber heat exchangers for use inside rotary kilns. A trefoil heat exchanger is a particularly common form.
Rotary kilns are typically built within steel cylindrical shells that have an intake end elevated a small height above a discharge end thereof. Aggregate material is fed into the intake end of these rotary kilns, the shell is rotated, the aggregate material processed and the material discharged out the discharge end. A trefoil heat exchanger is conventionally built at some distance along the axis of the cylindrical shell inwardly from the intake end.
Preferred constructions of trefoil heat exchangers are built of refractory brick and are mortared and keyed such that they will stay in place upon rotation. While integral castable trefoil constructions have been tried, it has been found that they are unable to sufficiently resist longitudinal flexure due to the rotation of the kiln. Precast trefoils crack due to this flexure. Because they are not keyed in place, pieces of the cracked precast trefoil are apt to fall out. On the other hand, trefoil heat exchangers that are constructed of a plurality of firebricks yield at their mortar joints. Pieces of them are less likely to fall out because of the keyed nature of their assembly.
Because the rotary kiln shell has an intake end elevated above the discharge end, all components inside the kiln exert a compressive force toward the discharge end. To resist this compressive force and thus prevent the sliding of kiln components towards the discharge end, buttress blocks are conventionally provided at the low end of the partition walls at their juncture with the shell perimeter. These buttress blocks are generally formed of conventional refractory materials that are weaker in their cross-breaking strength than in their crushing strength. Shear forces develop in them as the heat exchanger applies compressive force, causing the cracking and the dropping-out of the buttress blocks. The failure of these buttress blocks leads to the downhill sliding of components of the heat exchanger, thereby reducing the heat exchanger's useful life. A need has therefore arisen for multiple-chamber heat exchanger buttresses that will not fail due to shear forces but will transmit the compressive forces placed on them by the heat exchanger partitions to other components toward the discharge end of the rotary kiln.